1. Field of the Invention
The invention relates to communication of data between a transmitter and a receiver. It is particularly applicable to communication systems where the data is transmitted over a time-variant or frequency-variant channel, such as in mobile communication systems or satellite communication.
2. Description of the Related Art
This invention addresses the bit to symbol mapping for higher order modulation schemes when the modulation scheme is switched between ARQ (re)transmissions, e.g. in a system employing link adaptation by Adaptive Modulation and Coding (AMC) as it is used e.g. in 3GPP HSDPA.
In higher order modulation schemes, a plurality of bits b1, . . . , bi, . . . , bn (generally n>2) is mapped onto one symbol. This plurality of bits can be expressed as a binary word (or vector of bits) b1 . . . bi . . . bn, wherein to each value of this word or vector a certain modulation state is assigned. This assignment is called the “mapping” of bits to symbols. The order number i specifies the position of a certain bit (digit) within this word or vector and is hence called the “bit position”.
For a detailed explanation of AMC, please refer to 3GPP, Technical Specification 25.308, “High Speed Downlink Packet Access (HSDPA)”, Overall description, Stage 2, v. 5.3.0, December 2002 and A. Burr, “Modulation and Coding for Wireless Communications”, Pearson Education, Prentice Hall, ISBN 0 201 39857 5, 2001.
More information about HSDPA in general is available from 3GPP TS 25.308, cited above, and from 3GPP, Technical specification 25.212; “Multiplexing and Channel Coding (FDD)”, v. 5.3.0, December 2002.
In European Patent Specification EP1293059B1 it has been shown that the bit to symbol mapping for ARQ can be tailored in order to improve the ARQ performance. EP1293059B1 discloses a method changing the bit to symbol mapping over retransmissions in order to equalize the bit reliabilities over retransmissions. However, EP1293059B1 provides only a solution for cases in which the same modulation scheme is used in the ARQ transmissions.
For systems employing AMC, switching of the modulation scheme between ARQ (re)transmissions may be useful, especially if the (re)transmissions are transmitted with different power levels or received at different SIR levels (e.g. due to channel state variations).
In US Patent Application US 2003021240A1 and in “Selective Retransmission for Partial Soft Combining”, 3GPP TSGR1#21, R1-01-0780, Turin, Italy, Jun. 27-Aug. 31, 2001, a scheme has been proposed, where the bits which are retransmitted with a different modulation scheme are selected according to their content (systematic or parity bit).
There is no solution available for controlling or equalizing the bit reliabilities over retransmissions when switching the modulation scheme.
For simplicity, the description focuses in the following on the initial transmission of a packet/symbol and the first retransmission of a packet/symbol. Moreover, without loss of generality it is assumed that for the retransmission a lower order modulation scheme (less bits per symbol) is used than in the initial transmission. (In other examples, the opposite may be the case, for example if the re-transmission is to comprise additional redundancy data.) For a typical system (e.g. 3GPP HSDPA), re-transmission with lower order modulation scheme means that the first retransmission carries fewer bits than the initial transmission, since the number of symbols transmitted per transmission attempt is constant. Additionally for simplicity, Partial Chase Combining as described in “Partial Chase Combining For Code Management”, 3GPP TSGR1#20, R1-01-0543, Busan, Korea, May 21-25, 2001, is assumed, i.e. the retransmission carries only bits, which have been transmitted in the initial transmission (no additional redundancy). This allows achieving diversity combining gain at the receiver by soft combining of the received data, e.g. by Log Likelihood Ratio (LLR) combining.
It has been shown in EP1293059B1 and in Ch. Wengerter, A. Golitschek Edler von Elbwart, E. Seidel, G. Velev, M. P. Schmitt, “Advanced hybrid ARQ technique employing a signal constellation rearrangement,” IEEE VTC 2002 Fall, vol. 4, pp. 2002 2006, 2002, that a reduction in bit reliability variations improves the decoding performance. For the assumptions given above, the following two issues for the transmission mode need to be solved in order achieve a small bit reliability variation and, therefore, a good decoding performance:
1. Selection of the bits for retransmission, i.e. which of the bits of the initial transmission should be retransmitted;
2. Selection of signal constellations and bit mappings (bit to symbol mapping) rules for initial transmission and retransmission.
As mentioned in the previous section, solutions for the first issue have been e.g. discussed for 3GPP HSDPA, where the bits for the retransmissions are selected based on their content (systemic bit or parity bit). In US 2003021240A1 and 3GPP TSGR1#21, cited above, a proposal for preferring the systematic bits is shown. The current specification (see 3GPP TS 25.212, cited above) supports either preferring systematic bits or parity bits depending on a parameter (redundancy version).
Issue 2 is not applicable to prior art systems (e.g. 3GPP2 HDR, 3GPP HSDPA):                In 3GPP2, switching between modulation schemes between retransmissions is not considered.        In 3GPP HSDPA, for the given assumption (retransmission uses a lower modulation scheme than initial transmission) switching is possible from 16 QAM to QPSK. In this case, a selection of the QPSK mapping is not possible, since only a single QPSK mapping is defined. Moreover, the typically used Gray QPSK mappings have identical bit-reliability properties in all bit-positions (all bits have equal bit reliabilities). Therefore, the choice of the QPSK mapping does not have an impact on the bit reliability variation after combining and no impact on the decoding performance.        